Stupid like one over the eight (7)
I believe the answer is:
asinine
'stupid' is the definition.
(thesaurus)
'one over the eight' is the wordplay.
'one' becomes 'isa' (I can't justify this - if you can you should give a lot more credence to this answer).
'over' says the letters should be written in reverse.
'the eight' becomes 'nine' (I've seen this in other clues).
'isa' in reverse letter order is 'asi'.
'asi'+'nine'='ASININE'
'like' acts as a link.
Can you help me to learn more?
(Other definitions for asinine that I've seen before include "Foolish, silly" , "Idiotic" , "Donkey-like" , "Silly like a donkey" , "'Mindless, vacuous (7)'" .)